Invertebrate Hormones

Invertebrate
Hormones
In many metazoan phyla, the principal
source of hormones is neurosecretory
cells, specialized nerve cells capable of
synthesizing and secreting hormones.
Their products, called neurosecretions
or neurosecretory hormones, are discharged
directly into the circulation,
and serve as a crucial link between the
nervous and endocrine systems.
Neurosecretory hormones occur in
all metazoan groups. An extensively
studied neurosecretory process in
invertebrates is control of development
and metamorphosis of insects. In
insects, as in other arthropods, growth
is a series of steps in which the rigid,
nonexpansible exoskeleton is periodically
discarded and replaced with a
new, larger one. Most insects undergo
a process of metamorphosis,
in which a series of juvenile stages,
each requiring formation of a new
exoskeleton, end with a molt.

Insect physiologists have discovered
that molting and metamorphosis
are primarily controlled by interaction of
two hormones, one favoring growth and
differentiation of adult structures and
another favoring retention of juvenile
structures. These two hormones are molting hormone or ecdysone, produced
by the prothoracic gland, and
juvenile hormone, produced by the
corpora allata (Figure 36-4). The structures
of both hormones have been
determined. Extraction from 1000 kg
(about 1 ton) of silkworm pupae was
required to show that ecdysone is a
steroid.

Ecdysone is controlled by prothoracicotropic
hormone or PTTH. This hormone is a polypeptide (molecular
weight about 5000) produced by
neurosecretory cells of the brain, and
transported by axons to the corpora
allata where it is stored. At intervals
during juvenile growth, release of
PTTH into the blood stimulates the
prothoracic gland to secrete ecdysone.
Ecdysone appears to act directly on the
chromosomes to set in motion changes
resulting in a molt, by favoring development
of adult structures. It is held in
check, however, by juvenile hormone,
which favors development of juvenile
characteristics. During juvenile life,
juvenile hormone predominates and
each molt yields another larger juvenile
(Figure 36-4). Finally output of
juvenile hormone decreases, allowing
final metamorphosis to the adult stage.

The precise location of brain hormone
in the brain of pupal tobacco hornworms
was revealed by N. Agui by delicatemicrodissection. Using a human eyebrow
hair, he was able to isolate the single cell in
each brain hemisphere that contained brain
hormone activity. Thus only two cells, each
about 20 µm in diameter,produce this
insect’s total supply of PTTH. In an age
when sophisticated instrumentation has
removed much of the tedium (and some
creativity) from research, it is refreshing to
learn that certain biological mysteries succumb
only to skillful use of the human hand.

Chemists have synthesized several
potent analogs of juvenile hormone,
which hold great promise as insecticides.
Minute quantities of these synthetic
analogs induce abnormal final
molts or prolong or block development.
Unlike chemical insecticides, they are
highly specific and ecologically benign.